A thousand years ago, in what we now call Northwest Europe (including Great Britain), people used to count in base 12 (the dozen). Because 12 is easily divisible by 2, 3, 4 and 6 (and, of course, 1 and 12).

As the English language formed, base-10 was becoming more common (as were Arabic numbers, with their base 10). The first number past the dozen got called third after ten (thirteen) and the sequence continued (14 = fourth after ten = fourteen).

Eleven and twelve, by that time, had been established under much older proto-germanic roots. The old english name (in the 800s) was endleofon.

My guess (don't bet your entire savings on this):

endleofon = one left... to reach the dozen?